1. Field of the Invention
This invention relates to rubber belts and, more particularly, to a rubber belt that can be used to transfer an image to an object.
2. Background Art
Electrophotography is one form of electrostatography used in duplication. One process for performing electrophotographic duplication is carried out as follows. The first step is a charging step which involves applying a uniform electrostatic charge to a surface of a photoreceptor. An exposure step is then carried out to irradiate the charged photoreceptor with a light source, such as a laser light, to charge a predetermined pattern. In a development step, an image is formed by attaching a toner with the electrostatic charge. In a transfer step, the toner, defining the image, is transferred to ordinary paper or a film that is oppositely charged. The image is then fixed by transporting the paper or film, with the attached toner, through a heated fixing roll. Thereafter, the remaining charge and toner are removed to allow the photoreceptor to be reused.
In a conventional duplicating apparatus, the above development step is carried out by applying toner through the electrostatic charge developed on a transfer drum in contact with the photoreceptor. Generally, this mechanism involves a large number of parts, among which are a transfer drum, rollers, a charger, etc. As a result, the equipment tends to become complicated and expensive. With a large diameter transfer drum, operation of the transfer drum at high speeds tends to cause ordinary paper in contact with the transfer drum to attach thereto and cause clogs or jams.
In recent years, transfer belts have commonly replaced transfer drums. Transfer belts are often incorporated to produce a space efficient design. Further, the transfer belts are able to effect image transfer at high rotational speeds.
In a typical transfer belt, a conductive powder, such as conductive carbon black, graphite, metallic powder, and the like, is mixed with rubber. Electrical conductivity results from the contact of the particles in the conductive powder with each other. However, it is extremely difficult to uniformly disperse the conductive powder in the rubber. As a result, the resistance tends to vary throughout the transfer belt. As a result, the desired volume resistance may not be maintained in the desired medium resistance range of 10.sup.8 to 10.sup.11 .OMEGA.cm.
It is also known to add carbon black to rubber to increase its modulus and thereby reduce the elongation of the belt. Carbon black is generally required to be added at a high density. However, at high density, the carbon black tends to be dispersed non-uniformly, again potentially resulting in non-uniform electrical resistance properties.
It is known to use silica to increase the viscosity of the rubber. However, this tends to reduce workability and the resulting rubber tends to be brittle.
It is also known to produce anti-static properties by mixing a surface active agent and a filler consisting of a metallic oxide in a conductive rubber composition.
However, to achieve desired anti-static properties using the metallic oxide, generally a relatively large amount thereof must be used. While this improves the anti-static characteristics of the rubber, the modulus and permanent distortion resistance tend to decrease. Further, the anti-static characteristics attributable to the use of a surface active agent may be temporary. Bleeding may produce stickiness on the transfer surface. Even after charge removal and cleaning steps are carried out, there may be some residual toner attached to the transfer surface.
It is also known to form a transfer belt using a rubber composition made by mixing rubber, such as ethylene-propylene rubber (EPR), ethylene-propylene-diene rubber (EPDM), natural rubber, SBR, and the like, with epichlorohydrin.
With belts having a rubber composition that is a mix of EPR or EPDM with epichlorohydrin, the problem of the toner clinging to the transfer surface is eliminated to at least a certain extent. However, because the volume resistance of EPDM is typically as high as from 10.sup.15 to 10.sup.18 .OMEGA.cm, the amount of EPDM that can be mixed is limited to an amount that produces a resistance in the normally desired range of 10.sup.8 to 10.sup.11 .OMEGA.cm. The resulting belt may have a low modulus, which may result in lack of a desired clarity of the transferred image.